Purpose/Objective(s): Prophylactic irradiation of microscopic lymph node disease in breast cancer plays an important role. Nonetheless, the localization of lymph node (LN) metastases with regard to radiation treatment fields is still not perfectly understood as microscopic disease is not visible on imaging and thus other anatomical structures (for e.g. pectoral muscles) are used to define CTV levels. This study was performed to compare the dose distribution in loco regional FDG-PET/CT positive LN metastases between conventional tangent (CoTa) field and high tangent (HiTa) field irradiation.

Materials/Methods: 235 patients with 596 loco regional FDG/PET-CT positive LN metastases were included in this study. All FDG/PET-CT images were imported into the treatment planning software and each of the 596 PET positive LN was contoured separately. The contoured structures were non-rigidly registered to a patient with standard anatomy. LN levels (axillary, internal mammary, supraclavicular) and a PTV were contoured in the standard patient according to RTOG guidelines. The CTV to PTV margin for the breast was 1 cm, with inclusion of the chest wall. For high tangent fields the upper PTV margin was extended until 2 cm below the humeral head. The PTV prescribed dose was 50 Gy/2 Gy. We analyzed the average dose (Dmean), V20, V40 and V50 in each of the 596 lymph nodes as well as in the whole lymph node level (LNL).

Conclusion: There is a significant difference in dose coverage of LN / LNL depending on the irradiation technique (conventional tangent fields vs. high tangent fields). Further, there is a discrepancy between the Dmean in the whole RTOG LNL and the Dmean of actual LN within the LNL. This points that “hot spot” regions for LN can be defined within the same level and that “normal” radiotherapy fields might involuntary cover the same better than it covers the whole level. Nonetheless, 14% of lymph nodes are completely outside the RTOG LNL.